FIG. 1 depicts the structure of a conventional liquid crystal display device. Generally, the liquid crystal display device includes a liquid crystal panel 10 which has two substrates and a liquid crystal material sealed therebetween, a backlight module 20 disposed under the liquid crystal panel 10, and cases 11 and 12.
The backlight module 20 is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel 10. There are several kinds of backlight modules 20 such as a direct backlight type (or direct type) and an edge light type.
FIG. 2, which is a cross-sectional view along line 2—2 of FIG. 1, depicts a direct type backlight module 21. The direct type backlight module 21 includes a housing 70 having a bottom surface 71. A reflective sheet 60 is disposed on the bottom surface 71 of the housing 70. Lamps 50, such as fluorescence cathode tubes, are disposed proximal to the bottom portion of the housing 70. A diffusing sheet 40 is disposed above the lamps 50. A prism sheet 30 is disposed above the diffusing sheet 40.
FIG. 3, which is a cross-sectional view along line 2—2 of FIG. 1, depicts a edge light type backlight module 22. The edge light type backlight module 22 includes a light guide 80 having an edge 81. A lamp 50 is attached to the edge 81 of the light guide 80. In other embodiments, several lamps are attached to edges of the light guide 80. The edge light type backlight module 22 includes a reflector 61 having a generally U-shaped hollow interior 62 with an opening 63. The lamp 50 is disposed in the U-shaped interior 62. The opening 63 of the reflector 61 is disposed at the edge 81 of the light guide 80. A reflecting sheet 60 is disposed below the light guide 80, and a diffusing sheet 40 is disposed above the light guide 80. A prism sheet 30 is disposed above the diffusing sheet 40. Because the lamp 50 is disposed at the edge of the light guide 80, the thickness of the LCD can be relatively decreased.
Compared with the edge light type backlight module, the direct type backlight module can provide higher brightness since the light therefrom can directly enter the display area of a display panel without light loss caused by a light guide. Further, the manufacturing cost can be reduced since a light guide is not used. However, the conventional direct type backlight module using cold cathode fluorescent lamps (CCFL) still has following disadvantages:                1. Poor color saturation: the spectrums of red (R), green (G) and blue (B) colors are so wide that the current color filters cannot filter out pure colored lights (referring to FIG. 4), and the color saturation is around 72 percent of the color gamma defined by the National Television System Committee (NTSC) standard.        2. Slow response time: the response time is confined to the luminescence mechanism of the lamp and therefore greater than several milliseconds;        3. Being greatly affected by ambient temperature: the cold cathode fluorescent lamp (CCFL) has highest luminescence efficiency only under a specific temperature, and its luminescence efficiency is greatly affected by ambient temperature;        4. High driving voltage: the driving voltage of the cold cathode fluorescent lamp (CCFL) is greater than 1000V; and        5. Being environmentally unfriendly: the cold cathode fluorescent lamp (CCFL) contains mercury (Hg).        
Therefore, manufacturers have developed a direct light-emitting-diode (LED) type backlight module for solving the above problems. Compared with the direct CCFL type backlight module, the direct LED type backlight module has following advantages:                1. Excellent color saturation: the spectrums of red (R), green (G) and blue (B) colors are so centralized that the current color filters can easily filter out pure colored lights (referring to FIG. 5), and the color saturation is almost 100 percent of the color gamma defined by the NTSC standard;        2. Fast response time: the response time is greater than several nanoseconds;        3. Being not affected by ambient temperature;        4. Low driving voltage: the driving voltage of the red LED is about 2V to 3V and the driving voltages of the blue and green LEDs are about 3V to 5V; and        5. Being environmentally friendly: the LED does not contain mercury (Hg).        
Accordingly, the direct LED type backlight module has been the main trend for LCD device.